This invention relates to nanotechnology which is the science of controlling and manipulating particles (atoms and molecules) smaller than 20 nanometers. A nanometer is approximately 75 thousand times smaller than the width of human hair, or about 3 to 8 atoms wide. Specifically this invention relates to the production of nanoparticles that self align and thus are capable of self assembly into useful devices.
Since the discovery of doped nanocrystals (DNC) in 1994 made from ZnS with Mn2+ as the dopant, (See R. N. Bhargava et. al. Physical Rev. letters 72,416 (1994)). Several applications of these class of DNC's have appeared (See U.S. Pat. Nos. 5,422,489, 5,422,907, 5,446,286, 5,455,489 and 5,637,258). The external luminescent quantum efficiency of ZnS:Mn2+ nanoparticles was measured to be about 20% as compared to best efficiency in the bulk material of about 16%. In all the earlier work on doped nanocrystals, the size of the host was estimated to be less than 10 nm for efficient generation of light. These materials were developed over several years for various applications. In all the applications and products, the light generated in the nanocrystals was associated with the dopant (also called an impurity or activator) while the absorption was associated with the host.
In recent years we have concentrated in preparing DNC materials that showed enormous increase in the light output as the size decreased below 5 nm (see R. N. Bhargava et. al. Phys. Stat. Sol. (b) 229, 897 (2002)) and references cited therein. Indeed it was projected in our earlier work and patents that the luminescent efficiency increases non-linearly as the size decreases. The earlier data and conclusions were based on aggregates of nanophosphors prepared under different conditions. Since we did not have the ability to isolate the sizes of the particles and measure the individual nanoparticles for the light output, we concluded that the efficiency critically depends on size, but the critical role of the nano-size host was not understood. Recent developments in the preparation and separation of the particles, along with microscopic-optical studies of individual nanophosphors had led to a greater degree of understanding of the role of a single atom in a nanoparticle. Several devices and applications and products now emerge from this newly found science of DNC. As it happens that we are controlling the properties of a single atom by caging it, we have renamed the DNC's as Quantum Confined Atoms (QCA). This application describes the properties of QCA and how the modulation of properties of a single atom yield next generation devices in nanotechnology.
What are QCA's
A single atom of the dopant (activator) is confined in a cage of a 2 nm to 5 nm size nanoparticle of the host compound (8 to 20 atoms in a linear chain) and is referred to as a quantum confined atom (QCA). This is schematically represented in the FIG. 1 where the atom is represented as a charged cloud, a correct quantum mechanical representation of an atom. When so confined, the QCA shows extraordinary changes in its optical and magnetic properties. These properties change non-linearly with decreasing size. For example, our research has led to the creation of 2 nm to 5 nm size luminescent phosphors with light output similar to phosphors of size 1,000 times larger.
Recently we have demonstrated that in QCA based nanomaterials, the efficiency of the light emanating from a single caged atom (ion) is the highest when the particle size is less than 5 nm. As the size decreases from 10 nm to 2 nm, the light form the caged atom increases non-linearly as shown in FIG. 2.
This has established for the first time that a single atom in the cage experiences the ‘quantum confinement’ effect and that generates efficient light. This discovery demonstrates that the properties of a single atom can be manipulated controllably, and will impact optical and magnetic devices and is expected to become a formidable branch of Nanotechnology. Furthermore, the QCA's produced herein show self aligning (self-organizing) properties which can lead to self assembling nanodevices which is a significant step as it moves nanoparticles from the laboratory to commercially useful devices.
In conventional usage, classical phosphors comprise a host compound and a small amount of impurities that are referred as activator (or dopant). The absorption of the excited emission occurs within the manifold of energy-states that are either from the host or a combination of the host and activator. The emission is the characteristic radiative transition associated with the activator. From this classical definition of phosphors, we can designate QCA based luminescent materials as nanophosphors since the host is nanosize and the light emission is generated by the atomic ion (QCA). The hosts and activators associated with bulk phosphors when prepared in the size of 10 nm or below, result in efficient nanophosphors. The hosts and activator combination that do yield efficient bulk phosphors with size in the range of 1 to 10 microns can also be prepared as efficient nanophosphors. Additionally the quantum confinement of the activator ion allows us to prepare new luminescent materials that otherwise do not yield high efficiency in bulk form.
In bulk phosphors, the optimum concentration of activator is about 1% of the ion it is replacing. The activator is distributed statistically in the bulk phosphor. This random distribution activator-ions lead to separation from each other anywhere from 3 to 30 lattice spacing which corresponds to approximately 7 Å to 70 Å for a typical lattice spacing of 2.3 Å (0.23 nm). In the case of QCA based nanophosphors, recent studies (M. D. Barnes et. al, J. Phys. Chem. B 104 6099, 2000; and A. P. Bartko et. al. Chemical Physics Letters 358 459, 2002.) suggest strongly that there is either one activator-ion or zero activator-on per nanocrystal. The probability of incorporation of the single activator-ion in ananocrystal critically depends on the preparative methods, the starting concentration of activator-ion with respect to the ion it replaces and the size of the host.
The relative concentration of activator ion in the host changes significantly as the size of the phosphor particle decreases from bulk size (>50 nm) to nanosize (<10 nm). The statistical random distribution is shifted to binary distribution (either 0 or 1 activator ion in one nanoparticle). For example, a ZnS nanophosphor of size 5 nm would contain approximately 8000 total atoms for a lattice spacing of 2.5 Å and a simple cubic-crystal. The number of Zn sites in the crystal would be 4000. If one of the Zn-sites is replaced by Mn2+ ion, the concentration of Mn2+ relative to Zn would be 1/4000=0.025%. In bulk ZnS: Mn2+ phosphors, the optimum concentration of Mn2+ ion relative to Zn-atom in the nanocrystal for best luminescent efficiency of Mn2+ emission is determined to be about 1%. This concentration is approximately 40× higher than estimated for the best luminescent efficiency in nanophosphors of ZnS:Mn2+. This suggests that the activator ion like Mn2+ is 40 times more efficient in a nanophosphor than in bulk size. The dopant atom should satisfy the criteria that the valence state of the dopant atom conforms with one of elements of the host compound that it replaces and that the ionic charge-state and the size of the dopant atom is compatible with the ion in the host it is replacing.
The present application is directed to the preparation and use of a class of nanoparticles called Quantum Confined Atoms or QCA's. A QCA is a particle of material comprising a plurality of host atoms in a nanoparticle of a size of less than 10 nm with a single atom of a dopant (or activator) confined within. The QCA's have unique luminescent and optical properties and thus can act as a very efficient nanophosphor which generate polarized light and can operate as a laser and nanomagnet. An anti-agglomeration coating surrounding the nanoparticles can prevent clumping and loss of the enhanced properties.